1. Field of the Invention
The present invention generally relates to a passive component in an electrical system, and more particularly to an over-voltage protection device and a method for manufacturing the over-voltage protection device.
2. Description of the Prior Art
Over-voltage protection devices are widely used in electrical systems and electrical communication equipment for preventing elements thereof from being damaged by abnormal voltage or electro-static discharge (ESD) surges. Conventionally, the over-voltage protection device, as a varistor, is parallel connected with an electric source and has a variable resistance for conveniently facilitating adjustment of current flow or voltage. When the over-voltage protection device is in a normal state, it has a large electric resistance whereby current will not flow through the protection device. When the voltage of the electric source goes higher than a critical voltage of the over-voltage protection device, the electric resistance of the over-voltage protection device decreases quickly such that the large voltage of the electric source is directed to ground through the over-voltage protection device so as to prevent the other electrical elements from being damaged by the large voltage.
There are many kinds of over-voltage protection devices with the gap discharge type being the most widely used, as an ESD suppressor. The gap discharge type over-voltage protection device has a gap between two metal electrodes, the gap having a dimension of about several micrometers. When a large voltage appears between the metal electrodes, air within the gap is ionized so as to conduct an electric current between the metal electrodes causing the large voltage to be directed to ground so as to prevent the other electrical elements from being damaged. The gap should be maintained free of material other than air to protect against reductions in the stability of the withstanding voltage properties.
A conventional manufacturing method of the gap discharge type over-voltage protection device is disclosed in Taiwan Patent Application No. 200807673, in which the metal electrodes are formed by lithography and electroforming processes with curved shapes and a gap therebetween of about 0.5-10 μm. A proximity aligner is usually applied in the conventional lithography process. When using the proximity aligner, a suitable distance should be maintained between the mask and the substrate to prevent pollution of the mask from contact with the substrate. However, increasing the distance between the mask and the substrate increases a probability of light refraction and/or decreases a perpendicularity characteristic between the end edge of the metal electrode and the substrate. Normally, when using a positive photoresist, a profile of the positive photoresist near the substrate will be narrower with the profile of the positive photoresist farther away from the substrate being wider. On the other hand, use of a negative photoresist results in a wider profile of the negative photoresist near the substrate and a narrower profile of the negative photoresist farther away from the substrate. The electrode, which is formed by such a photoresist of poor perpendicularity, will also have a profile of poor perpendicularity. Therefore, the withstanding voltage properties of the over-voltage protection device will be unstable. Besides, the substrate used in the conventional manufacturing method is a thin aluminum oxide substrate which is made by a high temperature sintering process. Therefore, the substrate is likely to suffer from smoothness-control complications and warpage problems, with the profile of the electrode commensurately being affected by such substrate deficiencies.
Other approaches for forming the gap between the electrodes include the diamond sawing process and the laser cutting process. These processes are disclosed, for example, in Taiwan Patent Nos. M336534 and I1253881, according to which the gap between the electrodes can be controlled to 10-30 μm. However, if the gap is formed by the diamond sawing process or the laser cutting process, there is a risk that protrusions or burrs can be formed on the end edge of the electrode. The roughness of the end edge of the electrode is affected by the protrusions or burrs, resulting in a decrease in the stability of the withstanding voltage properties owing to the protrusions or burrs formed on the end edge of the electrode.